Pneumatic torque motor unit

ABSTRACT

The disclosure concerns pneumatic torque motor apparatus for resisting rotation of a shaft in opposite directions from a null position. The resisting torque is developed by a pair of seriesconnected calibration springs which are loaded by the piston of a pneumatic motor, and which act upon a drive rod which engages the shaft through flat bearing surfaces on an attached yoke and cooperating knife edges carried by a rocker member fixed to the shaft. One end of the drive rod is guided for reciprocation within the piston by a low friction head, and its tilting movement is limited by cooperating guiding surfaces on the yoke and rocker member. The head and a low friction annular member interposed between the two springs serve as seats for the calibration springs.

United States Patent Engle [451 June 6,1972

[54] PNEUMATIC TORQUE MOTOR UNIT [72] Inventor: Thomas H. Engle, Cape Vincent, N.Y.

[73] Assignee: General Signal Corporation [22] Filed: Oct. 9, 1970 [21 Appl. No.: 79,424

[52] US. Cl. ..92/1l3, l88/32l, 308/2 R 1,928,930 10/1933 De Juhasz ..308/2R &

a i I I g, i i i I Primary Examiner-George E. A. Halvosa AtrorneyDodge and Ostmann [57] ABSTRACT The disclosure concerns pneumatic torque motor apparatus for resisting rotation of a shaft in opposite directions from a null position. The resisting torque is developed by a pair of series-connected calibration springs which are loaded by the piston of a pneumatic motor, and which act upon a drive rod which engages the shaft through flat bearing surfaces on an attached yoke and cooperating knife edges carried by a rocker member fixed to the shaft. One end of the drive rod is guided for reciprocation within the piston by a low friction head, and its tilting movement is limited by cooperating guiding surfaces on the yoke and rocker member. The head and a low friction annular member interposed between the two springs serve as seats for the calibration springs.

3 Claims, 4 Drawing Figures PATENTEDJuu 6 I972 INVENTOR THOMAS H. ELM G LL:

ATTORNEYS PNEUMATIC TORQUE MOTOR UNIT BACKGROUND AND SUMMARY OF THE INVENTION Applications Ser. No. 761,656, filed Sept. 23, [968, now U.S. Pat. No. 3,528,709; Ser. No. 794,122, filed Jan. 27, 1969, now U.S. Pat. No. 3,536,360; and Ser. No. 807,295, filed Mar. 14, 1969, now U.S. Pat. No. 3,536,361, disclose self-balancing control systems in which input and feedback signals, in the form of torques, are algebraically combined by a rotary comparator shaft which actuates two pilot valves arranged to effect corrective action through main supply and exhaust valves. One of the torques is developed by a pneumatic torque motor unit including a calibration spring whose loading is varied by an air motor and which acts upon a drive rod which, in turn, engages the comparator shaft through an attached yoke and a pair of knife edges on a rocker arm fixed to the shaft. As illustrated in said patent applications, the piston contains a large axial bore which receives and guides an enlarged head at one end of the drive rod, and an aligned smaller bore whose wall engages the rod itself. This two-point guiding arrangement minimizes tilting of the drive rod, and thus keeps within tolerable limits variations in the moment arms of the forces applied to the rocker arm through the knife edges. In practice, however, the lower guide afforded by the piston was eliminated in order to reduce friction, and the drive rod was restrained laterally by causing the knife edges 'on the rocker am to seat in small depressions formed in the cooperating bearing surfaces of the yoke. Although this arrangement met with moderate success, it was found that vibrations sometimes caused the knife edges to move out of the depressions and permit intolerable tilting of the drive rod.

' The object of this invention is to provide an improved pneumatic torque motor unit in which excessive tilting of the drive rod is reliably precluded and frictional forces are kept to an acceptable low level. According to the invention, tilting of the drive rod is limited first, by the interaction between the rod head and the wall of the bore in the piston, and second, by the interaction between closely spaced cooperating guiding surfaces on the rocker member and the yoke. This guiding arrangement introduces less friction than the second piston guide employed in the units of the above noted patent applications, and it allows use of flat bearing surfaces on the yoke. Therefore, the new unit eliminates the disadvantages of both of the prior designs.

The preferred embodiment of the invention utilizes a calibration spring comprising two separate sections arranged in series. This feature reduces lateral deflection of the spring, and thus minimizes rubbing between the spring and the wall of the surrounding bore in the piston. Moreover, in this version, the head of the drive rod and the intermediate seat for the two sections of the calibration spring are made of a low friction material. This measure further reduces the friction level of the unit.

BRIEF DESCRIPTION OF THE DRAWING taken on line 22 of DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The illustrated pneumatic torque motor unit is especially adapted for use in the system of application Ser. No. 794,122, and serves to apply to comparator shaft 1 l a movement-resisting torque which is proportional to the difference between the pneumatic pressures supplied to its ports 12 and 13. Unit 10 includes a multi-part casing 14 formed to provide a pair of cylinders 15 and 16 of equal diameter which receive the heads 17 and 18, respectively, of a reciprocable piston assembly 19. Piston head 17 responds to the pressure in a working space 2! which communicates with port 12, and piston head 18 is subjected to the pressure in an opposed working space 22which communicates with port 13. It is assumed that the system associated with unit 10 supplies port 13 with air at a pressure equal to or less than that prevailing at port 12; therefore, since heads 17 and 18 have equal effective areas, assembly 19 will be urged upward by a force proportional to the difference between the two pressures. This movement of assembly 19 is opposed by a relatively stifi meter spring 23 so chosen that, for any given pressure differential within the design range, assembly 19 will assume 'a definite position along its path of travel.

The heads 17 and 18 of assembly 19 areinterconnected by a central stem 24 formed with an axial bore 25 which receives the upper end of drive rod 26 and in which are mounted the two sections 27 and 28 of the calibration spring. The spring sections 27 and 28 are arranged in series; the section 27 reacting between an annular seat 29 supported on snap ring 31 and the lower side of an intermediate seat 32, and the section 28 reacting between the upper side of intermediate seat 32 and a head 33 held captive on drive rod 26 by a screw threaded into its upper end. Head 33 and intermediate seat 32 are made of TEFLON polytetrafluoroethylene resin, or other comparable low friction material, and are guided for free sliding movement by the wall of bore 25. Except for the interaction between head 33 and the wall of bore 25, drive rod 26 is unrestrained against tilting movement by casing 14 or any of the components contained therein.v The upward force which calibration spring 27, 28 exerts on drive rod 26 varies directly with the deflection of meter spring 23,. and consequently is proportional to the differential between the pressures in working spaces 21 and 22. The sections 27 and 28 are so chosen that the calibration spring exerts little or no force on drive rod 26 when the pressure differential isa minimum and piston assembly 19 is in its lowest position, while, on the other hand, it applies a definite maximum force when the differential is a maximum and assembly 19 is in its uppermost position.

At its lower end, drive rod 26 carries a hardened steel yoke 34 which partially encircles a hardened, stainless steel rocker wheel 35 mounted on and keyed to rotate with comparator shaft 11. Yoke 34 includes intumed legs 36 and 37 which project into a recess 38 fon'ned in wheel 35 and are held captive therein by an annular disc 39 which overlies the front face of the wheel and is held in place by a nut 41 screwed onto the threaded end of shaft 11. The'legs 36 and 37 have slight axial clearance in recess 38 in order to preclude binding and insure free rotation of the wheel relatively to the yoke-The yoke legs 36 and 37 are provided with flat bearing surfaces 42 and 43 which lie in a common plane normal to the axis of drive rod 26 and which, in the illustrated null position of shaft 11, are engaged by knife edges 44 and 45, respectively, formed on wheel 35 at its periphery and spaced equally from the axis of shaft 11. One or the other of the knife edges 44 and 45 will engage the associated bearing surface 42 or 43 on yoke 34 as shaft 11 rotates from the illustrated position, so the upward directed force exerted on drive rod 26 by calibration spring 27, 28 always will be efiective to develop a resisting torque on the shaft. This restoring torque can be made to increase or decreme slightly with movement of shaft 11 depending upon the, relationship between the null position of the knife edges 44 and 45 and a plane which contains the shaft axis and is normal to the axis of rod 26. As shown in FIG. 2, the two knife edges are located above that plane in the null position of shaft 11; therefore, the moment arm of the point of contact between the active knife edge 44 or 45 and its associated bearing surface 42 or 43 will increase slightly as the shafi rotates. This arrangement effects a graduated increase in the restoring torque as the shaft is displaced and tends to increase the stability of the instrument in which unit 10 is employed-On the other hand, if the knife edges are positioned below the specified plane in the null position, the moment arm of said 3 point of contact, and consequently the magnitude of the restoring torque, will decreaseslightly as shaft 1 1 rotates from null'position. This effect, of course, tends to decrease stability. The arrangement selected for a particular case depends upon the combined effect of the stability-influencing characteristics of the other components of the instrument. 7

The restoring torque developed by unit 10 biases comparator shaft 11 to a position in which both of the knife edges 44 and 45 bear against the associated yoke surfaces 42 and 43, so obviously tilting of drive rod 26 in a plane normal to the shaft must be closely limited in order to insure that this position corresponds to the null position of the shafi. In the improved torque motor 'unit 10, the upper end of drive rod 26 is centered in bore 25 by head 33, and tilting movement is limited exclusively by the interaction between the surfaces 46 and 47 on yoke 34 and wheel 35, respectively. Both of these surfaces are cylindrical; the surface 47 being centered on the axis 48 of shaft 1 l, and the surface 46 being centered on a parallel axis 49 which lies in a common plane containing axis 48 and the axis of bore 25. The radii of surfaces 46'and 47 are selected so that thereis a small clearance of a few thousandths of an inch between each knife edge 44 or 45 and the adjacent intersection of the associated bearing surface 42 or 43 and cylindrical surface 46 throughout the range of angular motion of shaft 1 1. This guiding arrangement precludes excessive tilting of drive rod 26 without creating undue frictional resistance to reciprocating movement of rod 26.

It should be noted that use of a dual section calibration spring 27, 28 is desirable only in cases where piston assembly 19 has a long stroke (e.g., 2 inches), and the diameter of bore 25 is too small to permit use of a single section spring which affords adequate resistance to buckling. Furthermore, it should be observed that the desirable characteristics of the illustrated spring arrangement can be approximated using a long one-piece spring, and a guide which slides either on rod 26'or on the wall of bore 25 and limits lateral deflection of a central turn of the spring.

Although the illustrated embodiment is designed for use in the system of application Ser. No. 794,122, it must be understood that the invention encompasses torque motor units which include only a single air motor and which are useful in systems of the kind described in applications Ser. Nos. 761,656 and 807,295.

I claim:

1. Pneumatic torque motor apparatus (10) for resisting rotation of a shaft (1 l) in opposite directions from a null position, the apparatus comprising a. a housing (14) formed with a cylinder (l5, 16) open at one end to the exterior and containing a reciprocable piston assembly (19) which responds to the pressure in at least one working space (21 or 22), b. the piston assembly having a central element (24) containing an axial bore (25); c. a drive rod (26) extending into the housing and carrying a head (33) at its inner end which is guided for reciprocation by the wall of said axial bore (25 the drive rod being otherwise unrestrained against tilting movement within the housing; a calibration spring (27,28) encircling the drive rod and reacting between the head (33) and the central element a yoke (34) fixed to the drive rod(26) at its exposed end and provided with flat bearing surfaces (42, 43) located at opposite sides of the shaft (1 l) and lying in planes normal to the axis of the drive rod; and v f. a rocker member (35) fixed to rotate with the shaft (11) and provided with knife edges (44, 45)'which engage said bearing surfaces (42, 43) in said null position of the shah,

g. the rocker member and yoke normally engaging each other solely through the knife edges and bearing-surfaces but having closely spaced cooperating guiding surfaces (46, 47) which limit tilting of the drive rod,

h. the cylinder l5, l6), piston assembly (19) and axial bore (25) having a common axis which intersects the axis of the shah (l1) and with which the axis of the drive rod is maintained in coincidence by said cooperating guiding surfaces (46, 47).

2. Apparatus as defined in claim 1 in which the cooperating guiding surfaces comprise a. a cylindrical outer peripheral surface (47) on the rocker member which is coaxial with the shaft (1 l and b. a cylindrical inner peripheral surface (46) on the yoke which has a larger diameter than said surface (47) on the rocker member and has a parallel axis lying in a plane containing the axes of the shaft (11) and the drive rod (26).

3. Apparatus as defined in claim 1 in which a. the calibration spring comprises two coil compression springs (27, 28) arranged in series and having adjacent ends seated on opposite sides of an annular seat (32) guided for sliding movement by the wall of the axial bore (25), the remote ends of the springs being seated, respectively, on the head (33) and the central element 24); and

b. the head (33) and the annular seat (32) are made of low friction materials. 

1. Pneumatic torque motor apparatus (10) for resisting rotation of a shaft (11) in opposite directions from a null position, the apparatus comprising a. a housing (14) formed with a cylinder (15, 16) open at one end to the exterior and containing a reciprocable piston assembly (19) which responds to the pressure in at least one working space (21 or 22), b. the piston assembly having a central element (24) containing an axial bore (25); c. a drive rod (26) extending into the housing and carrying a head (33) at its inner end which is guided for reciprocation by the wall of said axial bore (25), the drive rod being otherwise unrestrained against tilting movement within the housing; d. a calibration spring (27,28) encircling the drive rod and reacting between the head (33) and the central element (24); e. a yoke (34) fixed to the drive rod (26) at its exposed end and provided with flat bearing surfaces (42, 43) located at opposite sides of the shaft (11) and lying in planes normal to the axis of the drive rod; and f. a rocker member (35) fixed to rotate with the shaft (11) and provided with knife edges (44, 45) which engage said bearing surfaces (42, 43) in said null position of the shaft, g. the rocker member and yoke normally engaging each other solely through the knife edges and bearing surfaces but having closely spaced cooperating guiding surfaces (46, 47) which limit tilting of the drive rod, h. the cylinder (15, 16), piston assembly (19) and axial bore (25) having a common axis which intersects the axis of the shaft (11) and with which the axis of the drive rod is maintained in coincidence by said cooperating guiding surfaces (46, 47).
 2. Apparatus as defined in claim 1 in which the cooperating guiding surfaces comprise a. a cylindrical outer peripheral surface (47) on the rocker member which is coaxial with the shaft (11); and b. a cylindrical inner peripheral surface (46) on the yoke which has a larger diameter than said surface (47) on the rocker member and has a parallel axis lying in a plane containing the axes of the shaft (11) and the drive rod (26).
 3. Apparatus as defined in claim 1 in which a. the calibration spring comprises two coil compression springs (27, 28) arranged in series and having adjacent ends seated on opposite sides of an annular seat (32) guided for sliding movement by the wall of the axial bore (25), the remote ends of the springs being seated, respectively, on the head (33) and the central element (24); and b. the head (33) and the annular seat (32) are made of low friction materials. 